Interfacial Atom‐Substitution Engineered Transition‐Metal Hydroxide Nanofibers with High‐Valence Fe for Efficient Electrochemical Water Oxidation

Abstract Developing low‐cost electrocatalysts for efficient and robust oxygen evolution reaction (OER) is the key for scalable water electrolysis, for instance, NiFe‐based materials. Decorating NiFe catalysts with other transition metals offers a new path to boost their catalytic activities but often suffers from the low controllability of the electronic structures of the NiFe catalytic centers. Here, we report an interfacial atom‐substitution strategy to synthesize an electrocatalytic oxygen‐evolving NiFeV nanofiber to boost the activity of NiFe centers. The electronic structure analyses suggest that the NiFeV nanofiber exhibits abundant high‐valence Fe via a charge transfer from Fe to V. The NiFeV nanofiber supported on a carbon cloth shows a low overpotential of 181 mV at 10 mA cm−2, along with long‐term stability (>20 h) at 100 mA cm−2. The reported substitutional growth strategy offers an effective and new pathway for the design of efficient and durable non‐noble metal‐based OER catalysts.


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transmission electron microscope (TEM) was performed by Tecnai G2 F20 S-TWIN operated at 200 kV. X-ray diffraction (XRD) measurements were characterized by Rigaku Ultima IV with Cu Kα irradiation. X-ray photoelectron spectra (XPS) measurements were performed on K-Alpha™+ X-ray Photoelectron Spectrometer System (Thermo Scientific) with Hemispheric 180° dual-focus analyzer with 128-channel detector and a monochromatic Al Kα irradiation. X-ray absorption spectra (XAS) were collected on the beamline BL07A1 in NSRRC (National Center for Synchrotron Radiation Research). The radiation by scanning a Si (111) double-crystal monochromator.

Electrochemical Measurements:
In a typical preparation of catalyst ink, 10 mg of each catalyst was blended with 1.0 mL Nafion ethanol solution (0.5 wt%) in an ultrasonic bath for 30 min. The carbon cloth (CC) and Ni foam (NF) were ultrasonically cleaned by acetone, ethyl alcohol, and ultrapure water in sequence for 20 min.
Subsequently, the CC was submerged in a 2 M H 2 SO 4 solution for another 12 h, and the nickel foam was sonicated in 2 M HCl for 20 min. To remove any additional acid, the CC and NF were rinsed with ultrapure water several times and then left dry in the air. Then a fixed volume of catalyst ink (10 mg mL -1 ) was pipetted onto the NF (loading: 1 mg cm −2 ), CC (loading: 1 mg cm −2 ), and glassy carbon electrode (loading: 0.255 mg cm −2 ).
All the electrochemical measurements were carried out in a conventional three-electrode cell using the Gamry reference 600 workstation (Gamry, USA) at room temperature. A commercial reversible hydrogen electrode (RHE) was used as the reference electrode, and the graphite rod was used as the counter electrode. The Ag/AgCl reference electrode calibrated with RHE in 1 M KOH was used as a reference electrode for long-time stability measurement. NF (1.0 × 1.0 cm 2 ) and CC The electrochemically active surface area was estimated by measuring the capacitance of the double layer at the solid-liquid interface with cyclic voltammetry. The measurement was performed S4 in a potential window of 0.82-0.92 V versus RHE, where the Faradic current on the working electrode was negligible. The series of scan rates ranging from 50 to 300 mV s −1 was applied to build a plot of the charging current density differences against the scan rate at a fixed potential of 0.87 V.
The slope of the obtained linear curve was twice of the double-layer capacitance (C dl ). [1] Electrochemical impedance spectroscopy (EIS) was carried out with a potentiostatic EIS method with a DC voltage of 1.495 V versus RHE in an Ar saturated 1.0 M KOH electrolyte from 100 kHz to 0.1 Hz with a 10 mV AC potential at 1600 rpm. The stability tests for the catalysts were conducted using chronopotentiometry at the constant working current densities of 10, 20, and 100 mA cm −2 .
The TOF values were calculated as the number of oxygen molecules evolved per active site per second based on the following equation: Where J is the current density (A cm -2 ) at the overpotential of 350 mV, A is the effective surface geometric area of the working electrode (0.196 cm -2 ), F is the Faraday constant, and n is the number of the active metal on the electrode. The TOFs data was calculated based on the weight content (from XPS) of the Fe in the catalysts.